ABSTRACT Bacterial endophthalmitis is a vision-threatening complication of penetrating eye injury and intraocular surgery, notably cataract surgery, the most common ophthalmic procedure performed in older populations in the United States. Approximately, 3 out of 1000 patients develop bacterial endophthalmitis after cataract surgery. As the aged population in the US is expected to grow dramatically, the number of cataract surgeries performed will also increase significantly, resulting in a proportional increase in the incidence of endophthalmitis. The visual properties of the retina are highly sensitive to inflammation-caused damage therefore, a rapid detection and clearance of invading pathogens is critical in minimizing retinal damage. The recent discovery presented in the preliminary data revealed that the retina responds to the TLR2 agonist Pam3Cys by producing the mediators of innate immunity, and that intravitreal injection of Pam3Cys, prior to bacterial infection, completely prevented the development of Staphylococcus aureus (SA) endophthalmitis in C57BL/6 (B6) mice. This leads to the hypothesis that TLR2 plays a critical role in retinal innate immune response to S. aureus and that activation and signaling through TLR determines the disease outcome. The objective of this proposal is to elucidate the mechanisms by which TLR2 activation prevents the development of SA endophthalmitis. Three specific aims are proposed: 1) To determine the role of TLR2 in retinal innate response against SA, and how TLR2 signaling is modulated by Pam3Cys pretreatment. The innate response will be tested in normal, TLR2 ligand, and the SA challenged B6 mouse retinas and cultured retinal (microglia, astrocytes, Muller and RPE) cells by assessing TLR-mediated cell signaling and production of proinflammatory cytokines/chemokines, 2) To determine the mechanisms of TLR2 ligand-induced stimulation of protective retinal innate immunity. Activation of TLR2 prior to infection induces protective mechanisms mainly by down- regulating proinflammatory (Th1) cytokines and up-regulating antimicrobial peptides (AMPs) in the retina upon subsequent bacterial challenge. TLR2-/- and MyD88-/- mice will be used to determine whether these mechanisms are TLR2/MyD88 dependent, active against other bacteria, and are effective in a mouse model of intravitreal injection-associated SA endophthalmitis. 3) To determine the role of cathelicidin related antimicrobial peptide (CRAMP) in retinal innate responses during SA endophthalmitis. The TLR signaling pathways involved in CRAMP induction and mechanisms by which CRAMP modulates bacterial clearance and preservation of retinal integrity, will be tested using cultured retinal cells and CRAMP knockout (Cnlp-/-) mice. Completion of these aims should provide insight into the understanding of the retinal innate response to microbial pathogens, and may lead to the identification of new therapeutic targets for preventing surgery- associated endophthalmitis.